Merging Multiple Maps Functionally

Question

I'm merging these maps :

val map1 : Map[String, String] = Map("four" -> "1", "three" -> "2");
val map2 : Map[String, String] = Map("four" -> "3", "three" -> "4");
val map3 : Map[String, String] = Map("four" -> "5", "three" -> "6");
val map4 : Map[String, String] = Map("four" -> "7", "three" -> "8");

using loops and mutable variables.

I'm not sure how to write a functional implementation ?

Complete code :

object tryout extends App {

val map1 : Map[String, String] = Map("four" -> "1", "three" -> "2");
val map2 : Map[String, String] = Map("four" -> "3", "three" -> "4");
val map3 : Map[String, String] = Map("four" -> "5", "three" -> "6");
val map4 : Map[String, String] = Map("four" -> "7", "three" -> "8");

val mapList : List[Map[String , String]] = List(map1 , map2 , map3 , map4)

val mergedMap: Map[String, List[String]] = Map("three" -> List("2","4","6","8") , 
                    "four" -> List("1","3","5","7"));

 val map = new scala.collection.mutable.HashMap[String,ListBuffer[String]]()
mapList.map(m => {

  m.map(m2 => {
    if(map.contains(m2._1)){
      val lb = map.get(m2._1).get
      lb += m2._2
    }
    else {
      map.put(m2._1, ListBuffer(m2._2))
    }
  })


})

map.foreach(m => println(m._1+","+m._2.mkString(",")))

}

Answer 1

This is a one liner but you can break it up as you please:

mapList.flatten.groupBy(_._1).mapValues(x => x.map(y => y._2))

Result:

res0: scala.collection.immutable.Map[String,List[String]] = 
Map(three -> List(2, 4, 6, 8), four -> List(1, 3, 5, 7))

In functional programming, explicit looping is avoided; we attempt to achieve the same using combinators like flatMap, map... Also, if you start thinking of a Map in Scala as a 2-tuple (which is what it is underneath) then dealing with them becomes much more intuitive in my opinion. This -> is just an implicit defined in the Prelude to make Map definitions easier on the eye.